The invention relates to a device for determining the energy state (state of charge) of an energy storing device of a mobile data carrier. Such a device can be used, for example, in connection with contactless identification systems.
Contactless identification systems use contactless transmission techniques, which can be based on electromagnetic transmission or transmission using light, infrared or ultrasound signals. Systems of this type are used, for example, in transportation systems, e.g., to identify persons or goods being moved. The necessary data are transmitted by a transceiver to a data carrier and back again over a contactless data link, e.g., an air interface. This contactless identification method also makes it possible to collect data while the data carrier moves past the transceiver, without the need for the data carrier to be inserted into, or swiped through a read/write device. Data carriers of this type are used, for example, as tickets with an electronically reloadable credit balance, such that the corresponding amount is automatically deducted when the means of transport is used.
German Publication DE 691 23 887 T2 discloses an IC card, which can detect a voltage drop in the built-in battery. For this purpose, the IC card is equipped with a data transceiver, a data processing unit, a charging unit, a comparator and a timer.
German Laid-Open Publication DE 100 54 970 A1 discloses a method for controlling the charging and discharging phases of a backup capacitor. In a circuit configuration, a constant current source is formed by a current-mirror circuit, and a comparator is used to compare the voltage on the backup capacitor with a band gap reference.
To enable the data carriers to be used for an indefinite period of time, the integration of chemical energy storing devices, e.g., batteries, is dispensed with in these units. The electric power required by the data carriers is instead picked up externally without contact, i.e., from a source of energy originating from the transceiver, e.g., an electric or magnetic field. Hence, suitable transmission and coding methods are required for the transceiver to communicate with such data carriers. On the one hand only certain frequency bands are typically released for the transmission of data, e.g., the ISM (Industrial, Scientific & Medical) frequency bands for industrial, scientific and medical applications. Possible national radio regulations may define, among other things, modulation bandwidths and field strengths to be complied with. On the other hand, the transmission and coding methods must also ensure the power supply of the electronics on the data carrier.
Such methods are described in ISO/IEC Standard 15693 Part 2, “Air Interface and Initialization.” Methods of this type enable a continuous power supply of the data carrier electronics, which is provided by the energy of the applied carrier frequency of the transceiver. To modulate the data to be transmitted, the carrier frequency is switched off only for a maximum time interval. Within this time interval, an energy storing device previously charged by the electric or magnetic field must be able to supply the power for the data carrier electronics. The temporary energy storing device used on the data carrier is generally a capacitor. The data are coded by switching off the carrier at defined positions within a cyclic time-slot pattern. Taking into account the aforementioned maximum time interval, the standard further defines the field strength limits for the sidebands produced by modulation at a certain carrier frequency. The height of the sideband modulation is determined on the one hand by the time ratio of the switched-on to the switched-off carrier frequency. In addition, further successive switching from the switched-on to the switched-off carrier frequency clearly contributes to the increase in the sideband modulation. The need to comply with the sideband limits defined in the standard leads to a maximum possible data rate.
Data transmission using contactless transmission methods can be undesirably influenced, however, by insufficient coupling. Such insufficient coupling can occur, for example, if a mobile data carrier moves very rapidly through a field or moves along the field boundaries where the energy transfer is low.
This can have drawbacks, for example, if a write process to a read/write memory of a mobile data carrier was started when the coupling between the mobile data carrier and the stationary read/write device was sufficient, but because of a movement of the mobile data carrier relative to the stationary read/write device, the energy storing device of the mobile data carrier cannot be adequately recharged. As a result, the power required for the write process may not be available in the mobile data carrier, so that the write process cannot be correctly completed.